The invention is based on a priority application EP 01 440 265.5 which is hereby incorporated by reference.
The invention concerns a method of equalizing a received signal, which is derived from a transmitted signal which is transmitted encoded via a transmission channel, and which includes a predefined sample bit sequence according to the pre-characterizing clause of claim 1.
The invention also concerns a receiver, which includes an equalization stage, and a receiving station which is equipped with it according to one of the accompanying claims.
In many telecommunication systems, message signals which include a predefined sample bit sequence are transmitted and received. There are both wire-connected and wireless telecommunication systems such as the Global System for Mobile Communications (GSM), in which such message signals are transmitted. Unlike the so-called “payload” part of the message signal, the sample bit sequences do not carry useful data, but signaling data which is used to improve the signal processing in the receiver and also to identify the message packets. These sample bit sequences are often called “training sequences”. If they are included at the start or middle of the message signal, they are also often called briefly “preamble” or “midamble”.
In the published patent application U.S. Pat. No. 2001/0004390 A1, a receiver for TDMA message signals, particularly TDMA radio signals, is described. This includes an equalizer in which iterative channel estimation is carried out (see, for instance, page 1, first three paragraphs). First, using the predefined sample bit sequence (“training sequence”), a first channel estimate of the transmission channel is carried out, and a first estimated channel pulse response is determined (see page 1, Section 0010 “generating an initial estimate of the pulse response of the radio channel”). Then, using the first estimated channel estimate, the received signal is equalized in an equalization stage of the receiver (see page 1, Section 0011). A second channel estimate is also made (see page 1, Section 0015 “generating an updated estimate”), so-called “feedback information” being used. The method is described there in more detail on the basis of FIG. 2, on page 3, Section 0038 ff, particularly Section 0041. According to that, the “feedback information” corresponds to an extended sample bit sequence (“sample sequence with more known symbols”), which includes bits of the predefined sample bit sequence (“training sequence”) as well as bits of a signal which is fed back through blocks “206” and “207”, these bits being adapted to the values of the original transmitted signal (see Section 0040 “are compatible with the original sample vector y”). This means that a reconstructed transmitted signal is formed by blocks “206” and “207” using the equalized received signal, and then, using an extended sample bit sequence, at least one second channel estimate (“updated channel estimate”) of the transmission channel is carried out and at least one second estimated channel pulse response is determined. This known iterative method improves the channel estimate directly by using an extended sample bit sequence, but the equalization itself is only improved indirectly, to the extent of the quality of the calculated channel pulse response.